Local Atomic and Electronic Structures of Pt Catalysts Using EXAFS Kenji KOBAYASHI, Fundamental Research Laboratories, NEC Corporation

k-kobayashi@ef.jp.nec.com

Recently, direct methanol fuel cells (DMFCs) have been much attracted as portable power sources for mobile applications. In

this cells methanol uses as fuel for the anode electrode and oxygen in air as oxidant for the cathode electrode. The methanol

oxidation and the oxygen reduction reactions are generally caused by the Pt–based electrocatalyst supported on carbon. In order

to improve the cell performance, it is important to realize exactly the characteristics of the Pt- catalysts because the catalytic activity

is affected from various factors. In this study we have investigated the

electronic structure of Pt 5d-band and the local atomic structures for Pt

catalysts by using XAFS.

11550 11560 115700.0

0.5

1.0

1.5

Abs

orp

tion (

arb.

units)

Energy (eV)

Pt/C (r=2 nm)Pt/C (r=3.4 nm)Pt/C (r=5.3 nm) Pt foil

Fig.1 The XANES spectra at Pt L3-edge

for Pt /Carbon catalysts and Pt foil.

XAFS measurements were performed at BL16B2 in Spring-8. The

XAFS data were collected around the Pt L3-edge by the transmission mode.

The used samples were 50 wt %Pt / carbon catalysts with a various particle

size of Pt. The average particle size is determined from XRD data. Fig.1

shows the Pt L3-edge XANES spectra for the Pt/carbon catalysts. It is

found that the white-line intensity increase gradually and its peak position

shift to high-energy side as the particle size of Pt become small. These

results indicate that the 5d band of Pt atoms on the surface is different from

that on bulk because the ratio of atoms on surface to total atoms increases in

Pt catalysts with a small particle size.

Local Atomic and Electronic Structures of

Pt Catalysts using EXAFS

Kenji KOBAYASHIFundamental Research Laboratories, NEC Corporation

PROGRAM OF 3rd SUNBEAM WORKSHOP

Introduction☆Direct methanol fuel cells (DMFCs)－ a future power source for portable devices.

☆Technological improvements

n Catalyst supporting electrodes

n Polymer electrolytes

n Fuels for portable devices

n Design for portable cell

O2H2 H+

ee

H2O

AirFuel (CH3OH)

Schematic drawing of DMFCs

Anode electrode (Ru-Pt/C)

Cathode electrode (Pt/C)

PROTON CONDUCTING MEMBRANES (Nafion)

Carbon-supported Pt or Pt-based alloys are effective electrocatalysts for methanol oxidation and oxygen reduction.

・Existence of optimum particle size Pt /C : 2 ～ 3 nm below 2nm → Decreasing of catalytic activity

・Increasing of surface area in small particle size → Improvements of catalytic activity

☆Electrocatalysts for DMFCs

We need overcome many problem. (ex. CO poisoning, overpotential, etc.)

Case1 : Particle size effect

・Catalytic activity is sensitive to surface structure. (arrangement, coordination, shape, electronic structure)

☆Characterization of Electrocatalysts by SR-XAFS

・SR-XAFS is a powerful tool to characterize the structure of the catalysts. EXAFS － coordination number, interatomic distance, disorderXANES － unoccupied state, chemical bond, oxidation state

Various characterization tools for surface structure(ex. TEM, XRD, XPS, FTIR etc.)

☆Purpose of this studyAs first step, particle size dependence of electronic and local structures for carbon-supported Pt electrocatalyts using EXAFS

Sample

・Pt loading 50, 80 wt%

☆Carbon-supported Pt electrocatalysts【preparation】

Colloidal method

Adsorption of Pt oxide colloids on carbon surface

H2PtCl6 solution

Pt oxide colloids

← Reduction and annealing in H2 atmosphere

Metallic Pt particles on carbon

← NaHSO3 + H2O2

← Carbon black

0 200 400 600 800 1000

5

10

15

20

part

icle

siz

e (n

m）

Temp. (℃)

50wt% Pt / C 80wt% Pt / C

30 40 50

1

2

3

Pt (200)

Pt (111)

Inte

nsity

(ar

b.un

its)

2θ (deg.)

50wt% Pt /C RT 700℃ 900℃

XRD of carbon-supported Pt electrocatalysts

☆Pt particle size－ was obtained from Scherrer formula. － increased above 700℃ in 50wt% Pt /C. － increased gradually with temperature in 80wt% Pt /C.

XAFS measurements

☆Sunbeam BL16B2 at SPring-8・Si (311) double-crystal monochromator + Rh coating mirror・beam size : 0.5 (V) ×2 (H) mm2

・Pt L3-edge・transmission method・ionization chamber gas: I0: N(75)+Ar(25) , I: Ar(100)

11550 11560 115700.0

0.5

1.0

1.5

Abs

orpt

ion

(arb

.uni

ts)

Energy (eV)

Pt/C (r=2 nm)Pt/C (r=3 nm)Pt/C (r=3.4 nm)Pt/C (r=5.3 nm) Pt foil

XANES of carbon-supported Pt electrocatalysts (1)

・Remarkable size effect

・In the small particle

－ white-line intensity increase

－ peak shift to higher energy side

White-line → Pt 5d-band

0 5 10 15 20

3.5

4.0

4.5

5.0

0.0

0.5

1.0

whi

te-l

ine

inte

nsity

(ar

b.un

its)

particle size (nm)

ratio

of

surf

ace

atom

－ ratio of surface atom

XANES of carbon-supported Pt electrocatalysts (2)

・Particle size dependence of white-line is corresponding to ratio of surface atoms.

・Pt 5d-band of surface atoms is different from that of bulk Pt.→ Increasing of Pt 5d-band vacancies in surface Pt atoms. → Charge transfer between Pt 5d-band and carbon support, adsorption atoms.

0 5 10 15 2011559.5

11560.0

11560.5

11561.0

11561.5

0.0

0.2

0.4

0.6

0.8

1.0

peak

pos

ition

of

whi

te-l

ine

(eV

)

particle size (nm)

rat

io o

f su

rfac

e at

om

White-line intensity White-line peak position

－ ratio of surface atom

5 10 15

0

wave vector (1/A)

0

0

0

EXA

FS χ

* k

3EXAFS of carbon-supported Pt electrocatalysts (1)

・Decreasing of EXAFS and FT amplitude in small Pt particle size.・Bonding with other atoms in surface Pt-site. (carbon supports or adsorping oxygen)

0 1 2 3 4 5 60

R (A)

0

0

0

FT (

arb.

units

)

Pt foil

Pt/C (r=5.3 nm)

Pt/C (r=3.4 nm)

Pt/C (r=2 nm)

Pt-Pt

Pt - ?

EXAFS signal FT of EXAFS signal

EXAFS of carbon-supported Pt electrocatalysts(2)Coordination number interatomic distance Debye-Waller factor

0 5 10 15 204

6

8

10

12

Nea

rest

-nei

ghbo

ur c

oord

natio

n nu

mbe

r　N

Pt particle size r (nm)

Pt/C cubooctahedron bulk Pt

0 5 10 15 20

0.275

0.276

Pt-

Pt

inte

rato

mic

dis

tanc

e　R

(nm

)

Pt particle size r (nm)5 10 15 20

0.065

0.070

0.075

0.080

Deb

ys-W

alle

r fa

ctor

σ2

particle size r (nm)

・E0 , λ was fixed to value obtained in Pt foil.・Reasonable CN calculated from Pt cluster model with cuboctahedron*.

* R.E. Benfield: J. Chem.Soc. Faraday Trans. 88(8) , 1107-1110 (1992).

【Coordination number (CN) for Pt-Pt bond 】・CN for the Pt-Pt bond decreased gradually below the particle size of 8nm. →This trend is consistent with that calculated from the simple cuboctahedron.

・The observed CN were reduced in small particles with respect to the calculated value. → We need to consider the cluster model with other geometry

or third cumulants term in Debye-waller factor.

【Interatomic distance】・Interatomic distance for the Pt-Pt bond also decreased gradually below

the particle size of 8 nm ・Existence of bonding with other atoms in surface Pt site.

EXAFS of carbon-supported Pt electrocatalysts(3)

SummaryRemarkable particle size dependence of local and electronic structures have been observed in carbon-supported Pt catalysts .

XANES

・Variation of white-line intensity and its peak position with particle-size

→ reflect to the electronic structure of surface Pt atom. Increasing of Pt 5d-band vacancies (charge transfer to neighbouring other atoms)

EXAFS

・Decreasing of CN and interatomic distance in Pt particle size below 8 nm.

・Existence of bonding with other atoms in surface Pt site.

Future plan

・Investigation for Pt/C below 2 nm

・In-situ XAFS measurements combined with voltammetric technique.・Application to other catalysts － Ru-Pt (anode), 3dTM-Pt (cathode)